Surface-enhanced resonance Raman spectra of acridine orange are recorded both on silver island films and on silver colloids, for solution concentrations as low as 10(-8) M. A detailed assignment is achieved by a comparison of the observed band positions with frequencies calculated by normal coordinate analysis. Band specific signal enhancement factors are used to obtain information on the adsorption geometry at the silver surface. Qualitatively similar surface-enhanced spectra are obtained when the acridine orange dye is intercalated into calf thymus DNA. The spectra are compatible with an adsorption geometry in which the axis of the double helix is oriented parallel to the silver surface. The structure and electronic properties of acridine orange are modeled by semiempirical quantum chemical calculations. Excellent agreement with respect to geometric parameters and the long-wavelength absorption maximum is obtained. Analysis of bond order changes upon excitation is used to discuss the band specific dependence of signal intensities on the excitation wavelength.